The invention relates generally to polymeric positive temperature coefficient (PTC) compositions and electrical PTC devices. In particular, the invention relates to polymeric PTC compositions containing low molecular weight polyethylene processing aids which are suitable for high temperature applications.
Electrical devices comprising conductive polymeric compositions that exhibit a PTC effect are well known in electronic industries and have many applications, including their use as constant temperature heaters, thermal sensors, low power circuit protectors and over current regulators for appliances and live voltage applications, by way of non-limiting example. A typical conductive polymeric PTC composition comprises a matrix of a crystalline or semi-crystalline thermoplastic resin (e.g., polyethylene) or an amorphous thermoset resin (e.g., epoxy resin) containing a dispersion of a conductive filler, such as carbon black, graphite chopped fibers, nickel particles or silver flakes. Some compositions additionally contain flame retardants, stabilizers, antioxidants, anti-ozonants, accelerators, pigments, foaming agents, crosslinking agents, dispersing agents and inert fillers.
At a low temperature (e.g. room temperature), the polymeric PTC composition has an ordered structure that provides a conducting path for an electrical current, presenting low resistivity. However, when a PTC device comprising the composition is heated or an over current causes the device to self heat to a melting temperature, a transition from a crystalline phase to an amorphous phase, resulting in a large thermal expansion, presents a high resistivity. In electrical PTC devices, for example, this resistivity limits the load current, leading to circuit shut off. In the context of this invention Ts is used to denote the xe2x80x9cswitchingxe2x80x9d temperature at which the xe2x80x9cPTC effectxe2x80x9d (a rapid increase in resistivity) takes place. The sharpness of the resistivity change as plotted on a resistance versus temperature curve is denoted as xe2x80x9csquarenessxe2x80x9d, i.e., the more vertical the curve at the Ts, the smaller is the temperature range over which the resistivity changes from the low to the maximum values. When the device is cooled to the low temperature value, the resistivity will theoretically return to its previous value. However, in practice, the low temperature resistivity of the polymeric PTC composition may progressively increase as the number of low-high-low temperature cycles increases, an instability effect. Crosslinking of a conductive polymer by chemicals or irradiation, or the addition of inert fillers or organic additives may be employed to improve electrical stability.
Attempts to improve the electrical stability have involved the use of high cure states, high molecular weight polymers and high levels of inert fillers. While these can significantly improve the resistance stability, the last two options adversely affect the processability of the material. Using higher states of cure adersely affects costs and voltage capability of the device.
In view of the foregoing, there is still a need for the development of polymeric PTC compositions and devices comprising them that exhibit a high PTC effect, have a low initial resistivity, that exhibit substantial electrical and thermal stability, and that are readily processable.
The invention provides polymeric PTC compositions and electrical PTC devices having increased voltage capabilities while maintaining a low RT resistance. In particular, the polymeric compositions also demonstrate a high PTC effect (the resistivity at the Ts is at least 103 times the resistivity at 25xc2x0 C.) and a low initial resistivity at 25xc2x0 C. (preferably 10 xcexa9cm or less, more preferably 5 mxcexa9 or less). The electrical PTC devices comprising these polymeric PTC compositions preferably have a resistance at 25xc2x0 C. of 500 mxcexa9 or less (preferably about 5 mxcexa9 to about 500 mxcexa9, more preferably about 7.5 mxcexa9 to about 200 mxcexa9, typically about 10 mxcexa9 to about 100 mxcexa9) with a desirable design geometry.
The polymeric PTC compositions of the invention, demonstrating the above characteristics, comprise an organic polymer, a conductive filler and a low molecular weight polyethylene processing aid. Optionally, but preferably, one or more additives selected from the group consisting of inert fillers, flame retardants, stabilizers, antioxidants, anti-ozonants, accelerators, pigments, foaming agents, crosslinking agents, coupling agents, co-agents and dispersing agents, by way of non-limiting example, may be employed. The compositions may or may not be crosslinked to improve electrical stability before or after their use in the electrical PTC devices of the invention. Preferably, the polymer component of the composition has a melting point (Tm) of 100xc2x0 C. to 250xc2x0 C.
The electrical PTC devices of the invention have, for example, the high voltage capability to protect equipment operating on line current voltages from overheating and/or overcurrent surges. The devices are particularly useful as self-resetting sensors for AC motors, such as those of household appliances, such as dishwashers, washers, refrigerators and the like. Additionally, PTC compositions for use in low voltage devices such as batteries, actuators, disk drives, test equipment and automotive applications are also described below.